I understand that serial # alone will not specifically tell you if a rifle has been issued to an army. Old US rifles (M1903, M1) will have special stampings on the stock that can indicate a rifle "may" have seen service...but it is not a guarantee...
BTW, those 2 models I mentioned are very easy to find serial number guides online that tell you when the rifle was made. In fact, I think there is M1 Garand Registry run by someone where thousands of rifles have been logged by their owners with some history available...